Peel and finely chop the shallots.
Heat a large pan. Tip in the cumin seeds and toast for 2-3 mins till popping and spicy smelling. Transfer them to a small bowl. Keep the pan on the heat and drizzle in 1 tbsp olive oil. Add in the shallots and cook for 5 mins on a low heat till softened.
Peel the skin from the ginger using the edge of a teaspoon. Trim and peel the carrots. Coarsley grate them. Finely slice the rhubarb. Crush the cardamom pods. Discard the papery skins and save the black seeds.
Add the ginger, cumin seeds and cardamom seeds to the pan. Stir together for 1 min. Add in the carrots and rhubarb. Stir to coat in the spiced shallots.
Tip in the red lentils. Crumble in the stock cube and pour in 700ml boiling water. Pop a lid on the pan and simmer for 20 mins.
Taste and season the soup. Blitz the soup till smooth using a stick blender or transfer to a food processor.